Linear beam electron devices are used in sophisticated communication and RADAR systems that generally require amplification of radio frequency (RF) or microwave electromagnetic radiation. A conventional klystron is an example of a linear beam electron device that has been used as a microwave amplifier. In a conventional klystron device, an electron beam originating from an electron gun may be caused to propagate through a drift tube that passes across a number of gaps, each gap corresponding to part of a resonant cavity of the klystron. The velocity of the electron beam is typically modulated by an RF input signal introduced at the first resonant cavity. The velocity modulation of the beam generally results in electron bunching due to electrons that have had their velocities increased gradually passing electrons that have had their velocities decreased. The traveling bunches of electrons generally correspond to an RF current in the beam, which induces electromagnetic energy in subsequent resonant cavities. The electromagnetic energy may thereafter be extracted from the last resonant cavities as amplified RF output.
Bandwidth and efficiency are both of some importance in klystrons. For example, the information rate of a signal a given klystron can amplify generally increases with the bandwidth. Also, the power consumed by the klystron typically decreases with increasing efficiency. The bandwidth of a klystron generally increases as the ratio of beam current to beam voltage increases, or rather, as the beam conductance is increased. This generally occurs when the load conductance across the output cavity and the loading conductances that the beam produces in the intermediate cavities are proportional to the beam conductance. Therefore, the quality factor (Q) for these cavities, which is a measure of the energy stored to the energy lost per cycle, generally decreases as the beam conductance is increased. Accordingly, bandwidth is also inversely proportional to Q.
The beam conductance may be determined by the perveance of the electron gun which produces the electrons and by the voltage at which the electron gun is operated. The perveance (K) is defined by the relationship between the beam current (I) and the beam voltage (V) as I=K√{square root over (V3)}. The perveance is generally 1E-6 to 3E-6 amperes per (Volt3)1/2 for the average klystron. The beam conductance   (      1    V    )may thus be expressed as       1    V    =      K    ⁢                  V            .      
In low-power klystrons, the beam voltage is usually comparatively low with the corresponding power output typically less than 1 kilowatt. One approach for increasing the bandwidth has been to increase the perveance since increasing the perveance generally increases the beam conductance and thus the bandwidth. However, this approach has had at least two disadvantages. First, if the perveance is made high, there is an adverse impact on the efficiency of the device because the space-charge forces in the beam generally increase making it difficult to tightly bunch the electrons of the beam. Second, as the perveance is increased at constant electron beam power, the beam voltage must generally be decreased. This typically results in a decrease in the electron beam velocity since the electron beam velocity is proportional to the square root of beam voltage. Furthermore, the dimensions of the cavity gaps along the beam must generally be held constant in terms of electron transit time in order to maintain good coupling of the cavity gap fields to the electrons. Therefore, the dimensions of these cavity gaps may be required to become extremely small in high frequency applications. Notably, there are substantial deficiencies associated with the prior art's ability to provide methods for the manufacture of klystrons suitably adapted for high-frequency operation. For example, U.S. Pat. No. 5,534,747 (Caruthers); U.S. Pat. No. 6,400,069 (Espinosa); U.S. Pat. No. 6,326,730 (Symons) and U.S. Pat. No. 5,796,211 (Graebner, et al.) generally describe conventional features of klystron design and construction, but do not address the problems of miniaturization and fabrication costs for the enablement of high frequency operation. Consequently, conventional klystrons have not generally found use at frequencies above 26 GHz due to limitations inherent to existing vacuum device processing.
Accordingly, it would be desirable to provide an efficient klystron for low-power, wide-bandwidth applications that could be easily fabricated. Furthermore, it would be desirable to provide a design methodology that would allow construction of various low-power klystrons for specific applications having relatively low output power and high efficiency, but at much higher frequencies and utilizing larger, more easily fabricated parts than may be found in klystrons of conventional design.